1. Field of the Invention
The present invention relates to electronic equipment which functions with power supplied by battery, and particularly, to electronic equipment having check device for battery life.
2. Description of the Related Art
In general, there exist various types of electronic equipment which function with power supplied by battery. In supplying power particularly in the electronic equipment usable outdoors, the battery life is a major issue.
Here, as a representative for electronic equipment usable outdoors, a camera will be exemplified and described. There exist a generally known digital camera, in which an object image incident through a photographing lens is photoelectrically converted by a solid state image pickup element such as CCD and the like, and this photoelectrically converted image signal is A/D converted to be recorded on a recording medium, and further, an image can be displayed by a built-in liquid crystal monitor.
Particularly, a single lens reflex camera with exchangeable photographing lens is required to have good operability and high-speed continuous photographing performance similarly to a silver film camera, pick up an image with high quality and have a wide brightness range of the object to be taken, for example. For that reason, it is necessary to adopt the image pickup element which has a large number of pixels and is highly sensitive. Furthermore, as compared with the silver film camera, a large scale electrical circuit using a large number of electrical parts such as an imaging circuit, image processing circuit, image display circuit and the like is added.
As a result, in the digital single lens reflex camera, a power consumption is large, and a battery capable of supplying sufficient energy is required. On the other hand, as the miniaturization and light-weight of the camera advances, it is increasingly difficult for the conventional primary battery or secondary battery to supply sufficient energy for driving the camera.
As a solution for such problem, an attention is paid to a compact fuel cell. The fuel cell is high in power generating efficiency as compared with the conventional generating system, and moreover, its waste is clean. In addition, since an amount of supplied energy is nearly several to several tens times that of the conventional battery per cubic content and weight, it is said that the fuel cell is useful as a power source for compact electronic equipment.
Hereinafter, a principle of power generation in the fuel cell will be described. The fuel cell supplies fuel gas containing hydrogen to a fuel electrode, and supplies oxygen gas containing oxygen to an oxygen electrode, thereby obtaining an electromotive force through electro-chemical reaction arisen at both electrodes. Hydrogen supplied to the fuel electrode is separated by catalytic agent into proton and electron. The separated electron moves to the oxygen electrode via an external circuit, and proton moves to the oxygen electrode through a solid high polymer film (high polymer electrolyte film). In the oxygen electrode, proton, electron and oxygen are combined to generate water and carbon dioxide.
The electro-chemical reaction arisen at the fuel cell will be shown. A formula (1) represents a reaction arisen at the fuel electrode, a formula (2) represents a reaction arisen at the oxygen electrode, and a formula (3) represents a reaction arisen at the entire battery.H2→2H++2e−  (1)(1/2)O2+2H++2e−→H2O   (2)H2+(1/2)O2→H2O   (3)
FIG. 16A is a top plan view showing one example of the fuel cell, and FIG. 16B is a front view thereof. This fuel cell takes oxygen used for reaction as an oxidant from the outside air, and therefore, the upper surface, under surface, and longitudinal side surface of a body frame 70 are provided with an air pit 73 for taking in the outside air. This air pit 73 serves to vent the generated water as moisture vapor and also serves to vent the heat generated by the reaction. The other short side surface of the body frame 70 is provided with an electrode 72 for drawing electricity.
On the other hand, the interior of the body frame 70 is composed of a cell portion 71 consisting of more than one cell including a fuel electrode 113, a high polymer electrolyte film 112, an oxygen electrode 111. It is further composed of a fuel tank 76 for storing a fuel, a fuel supply route 75 connecting the fuel tank 76 and each cell portion 71, and a pressure sensor 77 for measuring the pressure of the fuel.
The fuel-cell cell thus configured has an electromotive force about 0.8 V and an electric current density about 300 mA/cm2, and for example, the size of an unit cell is specified to about 1.2 cm×2 cm. When eight pieces of this fuel-cell cell is connected in series, the output of the entire battery becomes about 6.4 V and 720 mA, resulting in about 4.6 W. In FIGS. 16A and 16B, while the fuel-cell cell is shown with two laminated sheets of the cells each having the same area, a large number of laminated sheets connected in series as described above can provide a high voltage.
The interior of the fuel tank 76 is filled with hydrogen absorbing alloys capable of absorbing hydrogen. Since the withstand pressure of solid high polymer film used in the fuel cell is 0.3 to 0.5 MPa, it is necessary to use the film within the range where the differential pressure with the outside air is 0.1 MPa.
As the hydrogen absorbing alloys having a property of 0.2 MPa in the release pressure of hydrogen at normal temperature, LaNi5 and the like are used. Since LaNi5 can absorb and desorb hydrogen of 1.1 wt %, the amount of hydrogen stored in the fuel tank 76 is 0.4 g, and energy capable of generating the power is about 11.3 W·hr, which is about four times that of the conventional lithium ion battery. On the other hand, when a hydrogen absorbing material exceeding 0.2 MPa in release pressure of hydrogen at normal temperature is used, it is necessary to provide a pressure reducing valve 78 between the fuel tank 76 and the fuel electrode 113.
Hydrogen stored in the fuel tank 76 is supplied to the fuel electrode 113 through the fuel supply route 75. The outside air is supplied to the oxygen electrode 111 through the air pit 73. Electricity generated by the fuel cell is supplied from the electrode 72 to the electronic equipment which is to be driven.
A part of each electrode contacting water is insulated so that the electrodes of the fuel cell are not energized through the water for electrolysis when being charged. One method of insulation is to coat a part not contacting the solid high polymer film of the electrode with insulating material.
In the fuel cell thus configured, when proton moves to the oxygen electrode through the solid high polymer film, and if the solid high polymer film is dried, an electric resistance value of the solid high polymer film is increased. As a result, the power loss in generation of electric power becomes large, and the power generating capacity of the solid high polymer type fuel cell becomes small, that is, the voltage that can be generated becomes low.
Thus, immediately after the fuel cell starts to supply power, the voltage that can be generated becomes low. Particularly, in the fuel cell left unused for a long period of time, the solid high polymer film is further dried, and the voltage that can be generated at a start-up time becomes much lower. In FIG. 17, an example of the discharge curve of the fuel cell left unused for a long period of time is shown. The axis of ordinate represents the voltage V, and the axis of abscissas represents the time T. In FIG. 17, at a start-up time, since the solid high polymer film is in a dried state, the voltage that can be generated becomes low. After that, as the power generation progresses, the solid high polymer film is gradually humidified by H2O generated by the reaction of the formula (2), and the power loss is reduced, and the discharge curve ascends. Then, when the supply of hydrogen stops, the discharge curve descends.
As a power source system using such solid high poly film type fuel cell, a method of humidifying the solid high polymer film to prevent the voltage that can be generated at a start-up time, from becoming low is widely under review. For example, according to the fuel cell device disclosed in Japanese Patent Application Laid-Open No. H09-213359, a water-holding material is disposed inside the fuel cell, and H2O generated when the power is outputted is stored in the water-holding material. Then, at a start-up time, hydrogen is let pass through the water-holding material, so that the humidifying of the solid high polymer film is performed.
Further, according to the control method and the control device of the fuel cell as disclosed in Japanese Patent Application Laid-Open No. 2003-234116, separately from the fuel cell, a secondary battery and a capacitor are provided, and at a start-up time, the power is supplied from the secondary battery and the capacitor.
Incidentally, to comfortably use the electronic equipment, it is necessary to accurately know a remaining power of the battery as a power source. Hence, a number of the electronic equipment are provided with a remaining amount detecting device of the battery capacity, and they are configured such that a voltage check (battery check) of the built-in battery be made at the operating time.
As a battery check, there is known a method in which the battery is applied with a load for the predetermined time (that is, the load is energized), thereby letting the battery voltage drop, and it is determined whether the dropped battery voltage is equal to or more than the predetermined level. As a result of performing the battery check, if the battery voltage is equal to or more than the predetermined level, the procedure proceeds to the next sequence operation, and the camera is activated. On the contrary, if the battery voltage is below the predetermined level, the camera is not activated. This battery check is performed, for example, when the half depression operation (operation for starting the photographing preparation such as photometry, distance measuring, and the like) of the release button of the camera or the main switch is performed or performed in the midst of photographing sequences (for example, immediately before a shutter charge is performed after the shutter completes a travel motion) of the camera.
In the equipment using the fuel cell as a power source which uses hydrogen supplied from a hydrogen gas cylinder as a fuel, as the remaining amount of hydrogen decreases, hydrogen pressure is lowered. Hence, there exists a technology as a known technology, in which the pressure inside the hydrogen gas cylinder and the hydrogen pressure discharged from the hydrogen gas cylinder are detected, thereby performing the detection of the remaining amount of hydrogen.
Further, as a method of the battery check at the side of camera which uses the fuel cell using fuel liquid such as methanol and the like as a power source, there exists a method of confirming the remaining amount of the fuel liquid from the outside of the camera by visual observation (for example, see Japanese Patent Application Laid-Open No. 2003-295284).
Further, as the remaining amount detecting device of the fuel cell, there are many devices proposed based on the technology of calculating the remaining amount of the battery capacity from the initial amount and the used amount. For example, in Japanese Patent Application Laid-Open No. H11-230813, there is disclosed a device for calculating the remaining amount of the battery capacity from the initial fuel amount and the used amount.
As described above, in the electronic equipment mounted with the fuel cell, the executing of the battery check is required. In this case, for example, in the remaining amount detection which is disclosed in Japanese Patent Application Laid-Open No. H11-230813, a flow meter and an ampere meter are required, and this causes a cost up and the like.
Hence, it becomes effective to perform the battery check by applying a load for the predetermined time to the battery (that is, the load is energized) to drop the battery voltage, and determining whether the dropped battery voltage is equal to or more than the predetermined level. If this technique is adapted, the configuration of the battery can be made simple, and the miniaturization of the electronic equipment is not hampered, thereby a cost up can be controlled.
However, as already described, if the fuel cell is used, the voltage at a start-up time is low, and therefore, when compared with the reference voltage at the start-up time, it is sometimes determined as an inhibit voltage level (voltage level dropping below the voltage by which the camera is operable). That is, there arises a problem that, despite of the fact that in reality the driving of the camera is possible, it is determined that the camera cannot be driven.
It is conceivable that the voltage at a start-up time is compensated by the technology disclosed in Japanese Patent Application Laid-Open No. H09-213359 and Japanese Patent Application Laid-Open No. 2003-234116, so that such a problem does not occur. However, in the fuel cell disclosed in Japanese Patent Application Laid-Open No. H09-213359, a space for disposing the water-holding material inside the fuel cell is required, and this makes further miniaturization of the fuel cell difficult. Further, since the water-holding material is required, this causes a cost up. While, in the control method and the control device of the fuel cell disclosed in Japanese Patent Application Laid-Open No. 2003-234116, separately from the fuel cell, the secondary battery and the capacitor are required, and this not only hampers the miniaturization of the camera, but also leads to the cause of a cost-up.